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12AX7 + 12AU7 preamp with feedback?

6V6dude

Member
2014-09-09 11:07 am
I'm looking to build new preamp section for SE 6V6 stereo amp but not sure what to go for. I've found this schema, looks like it has a feedback? I marked it in red line. What is the purpose of that?
Currently I have just single stage basic 12AX7 tube with 1.2k on cathode and output from anode similar to the 12AX7 section in this schema and it works fine. Would adding the 12AU7 and the "feedback" make any difference? I don't need more gain so I'm not sure what the 12AU7 does apart maybe from output impedance.
Thanks for any info
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ashok

Member
2002-06-06 4:43 am
3RS
The 12AU7 provides additional gain. You then provide feedback to the input stage to reduce gain and output impedance.Lowers distortion too.
The input resistor is bootstrapped here and so it's effective impedance will be very high. Hence you can get away with an ( affordable !) high quality low value input capacitor.Maybe even 0.01 uF ! Here they have shown 0.1uF . That is enough with the 1Meg input resistor connected to ground instead of the bootstrap point between R2 and R3. But if you connect R1 to ground you do not need C1 !
Here you have dc feedback too. It doesn't control the overall dc points but it does affect the input stage bias point.I guess the resistors have been suitably selected to enable that.If you sim it you can confirm that.
Maybe you can just use one 12AU7 alone ? The second stage by itself ! It should have enough gain and low enough output impedance to work fine with your tube SE stage ? Distortion will be higher but acceptably low.
Input going direct to grid of 12AU7 ( eliminate C3). Retain R12, R13 , R14 and C5 and if you want R18 . Nothing else needed.

12AX7 has a rated gain of 100 and higher output impedance (Rp) and 12AU7 has about 20 and much lower output impedance. In circuit gain will be less.
 
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6V6dude said:
I'm looking to build new preamp section for SE 6V6 stereo amp but not sure what to go for.
What do you mean by "preamp section"? If you mean 'preamp' then why do you think you need a preamp? If you mean 'voltage amplifier' (i.e. the first part of a power amplifier) then the circuit you showed may or may not be suitable, although a simpler circuit may be better.
 

ashok

Member
2002-06-06 4:43 am
3RS
Actually I think you really don't need to do anything ! Your 12AX7 is doing the job. A 12AU7 will reduce gain ! Do you want to do that ? A two tube pre-amp probably will not sound any better than your existing 12AX7 in spite of of the two tube ckt having less distortion due to negative feedback. The SE has enough distortion to mask that.
You could put in better coupling (poly prop .....MKP....film) caps at the input ( if you use one ) and the output of the 12AX7 stage.
 

6V6dude

Member
2014-09-09 11:07 am
Thanks a lot for your replies! Glad the feedback got confirmed. I don't like FB on hifi amps so won't bother with this schema than. I had FB on it when I build the amp (10 years ego) connected to OT but I didn't like the sound. It was kind of constricted even tho it sounded little cleaner, so I got rid of it.
I'll probably keep the single stage 12AX7 with some minor mods, maybe lowering the gain a bit. I did try 12AU7 yesterday, the gain was about right but it sounded a bit flat for my liking. The 12AX7 seems to sound fuller and more complete to me, more on the edge might be the right description. I hope to finalize my decision on it tonight so will send update.
Why I'm I rebuilding this amp? Mainly because I'm changing power trafo and changing from tube rectifier to solid state one. The original trafo was putting like 800V on startup, just too hard for the tubes before it all came down to 300V plus the trafo was made in1920's so really old, I felt it could die anytime. Lasted 10 years but I've never been comfortable with it being on while I'm away from the room.
 

ashok

Member
2002-06-06 4:43 am
3RS
You cannot just plug in the 12AU7 and compare it. The 12AX7 typically operates around 1mA current. The 12AU7 operates at a few mA say 3 to 4 mA . So with the resistors you have on board the 12AU7 will not be at the right operating point. There should be no change in HF response. But the 12AU7 will have much lower gain. With a 1 meg load the gain can be almost a 1/4 of the 12AX7. With a 47K load it might be 1/2. But at low loads the 12AX7 will distort more.
You could use the 12AU7 with different plate and cathode resistors. Suspect that's all the gain you will need.
 

6V6dude

Member
2014-09-09 11:07 am
Yeah, I just stuck the 12AU7 in place of 12AX7 for a quick test and yes it was only reading 70V. You're right it would need to be setup but at this point I'll probably go with 12AX7 because I have those new. But I'll try the 12AU7 properly anyway once I'll rewire the amp first which is what I'm doing right now.

But I'd like to ask, do you think that inside a dual triode like 12AX7 the components are matched or if I used two separate tubes of same brand it would not matter? I'm asking mainly because I have now 3 empty sockets left because I got rid of rectifier, tone control 12AX7 circuit and VU magic eye tube. I thought might put back the magic tube and have one triode in each socket to make it look decent. Capacitor can will go instead of the rectifier tube and that way I won't be left with holes in the chassis.
 
6V6dude said:
I don't like FB on hifi amps so won't bother with this schema than.
Feedback is an essential part of all hi-fi amps. In all cases it is intrinsic to triodes; in most cases more has to be added via the circuit.

I had FB on it when I build the amp (10 years ego) connected to OT but I didn't like the sound. It was kind of constricted even tho it sounded little cleaner, so I got rid of it.
OK, you don't like 'clean' sound; you don't like hi-fi. Not everyone does. Sound is a matter of taste. Anyway, you are unlikely to get hi-fi from an SE 6V6 - 3-4W at 10% distortion is not hi-fi.

But I'd like to ask, do you think that inside a dual triode like 12AX7 the components are matched
No they are not, unless you are lucky or paid to have a match found.
 
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ashok

Member
2002-06-06 4:43 am
3RS
Yes, as DF96 said. The two sections are not actually matched usually. But some could measure quite close to each other. Some tube suppliers can supply matched sections. That just involves taking several tubes and measuring each section and picking those that are close to each section ( in one tube ).

I buy tubes from Singapore where I always get tubes with the measurement written on the outside for each section. You get them from the US also. Some charge extra for that.
 

6V6dude

Member
2014-09-09 11:07 am
OK, you don't like 'clean' sound; you don't like hi-fi. Not everyone does. Sound is

You've hit the nail right on the head! I like my sound little dirty. But it must be from 6V6 or 6L6, other tubes just don't do it for me. While I do have some nice clean SS amps, I can't listen to them for hours like I can to tube. To me tube is very soothing, round and soft, I've loved that since I was a small kid.

Anyway, I might go with two single stage 12AX7's to fill that 2th empty socket and be done with it. I did try 6SN7 this afternoon but it sounded too sterile to me. Strangely enough I have another PP 6V6 amp with 6SN7 for preamp and that sounds good. I guess it all comes down to combination of output trafos as well and how it all matches together = final end result.
 

6V6dude

Member
2014-09-09 11:07 am
Well I've spend the day trying things properly. Tried 12AU7 and 6SN7 again but there is not enough gain from single stage for lot of the 60's bellow 0db music so I'll go with single 12AX7.
Tried doing feed back from OT as well but didn't like it. Somehow the sound doesn't fill the room at low volumes as without it. I'm just guessing it gets rid of the harmonics?
Other than that the amp sounds great and has immensely deep bass, I can only guess at 15hz at least and that's driving big Advent speakers. EVen my SS amps don't seem to reach that low even tho the so called spec say so.
Thanks for the tips to all.
 
Yes, feedback reduces harmonics generated in the amplifier. It does not, of course, remove harmonics and overtones already present in the music. Feedback reduces gain too so you would need to increase the volume control.

15Hz bass is unlikely unless you have an exceptionally good (and humungously large and expensive) OPT. However, SE without feedback is likely to have quite high output impedance so you will get a bass peak at the bass resonance of the speaker. This may sound like deeper bass, but it is actually really 'one note' bass. Note that 15Hz is barely audible to most people. Given sufficient volume (unlikely with a 6V6 SE) you are more likely to feel 15Hz than hear it.
 

6V6dude

Member
2014-09-09 11:07 am
Yes true, the 15hz for me is on the border of feeling and hearing but I can still hear it as I did some listening tests using tone generator. Unfortunately I can't measure the amp because I only have software oscilloscope and that only does 20-20khz. Also by looking at the woofer the low vibration is visible. Ok I know, just a speculations.
But for having the extra bass, I think there is a tiny dip at about 8-10k. It's a very soft, rounded sound. The OTs are from 60's Japanese amp/receiver that had 6GW8 tubes so there might be some impedance mismatch. 6GW8 is 5900 and 6V6 is 5000 at 250V and 8500 at 315V by my tube data. I'm running the 6V6 at 300V but I've tried running it at 250 to try to get closer to the 5000 and apart from lower power it didn't sound any different. That's by my crude assumptions anyway.
 
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